1. Field of the Invention
The present invention relates to a semiconductor laser device and a method for fabricating thereof, and in more detail a semiconductor laser device having a structure capable of preventing non-emissive failure due to short circuit and a method for fabricating such device.
2. Description of the Related Art
A visible light semiconductor laser device having a stacked structure on a GaAs substrate, wherein an active layer is sandwiched by cladding layers made of AlGaInP or GaInP, has an oscillation wavelength between 630 nm and 690 nm, and attracts a good deal of attention as a light source for an optical pickup used in an optical disc drive.
A structure and fabrication method of a conventional AlGaInP-base visible light semiconductor laser device will be explained hereinafter referring to FIG. 6. FIG. 6 so shows a cross-sectional view of the substrate showing a structure of an AlGaInP-base semiconductor laser device.
An AlGaInP-base semiconductor laser device 10 has on a GaAs substrate 12 a stacked structure comprises a lower cladding layer 14 made of n-AlGaInP, an active layer 16, an upper cladding layer 18 made of p-AlGaInP, and a contact layer 20 made of p-GaAs, and all layers are epitaxially grown in this order.
An additional semiconductor layer such as light confining layer may optionally be provided between the upper cladding layer 18 and the contact layer 20. Also a buffer layer made of compound semiconductor may optionally be provided between the GaAs substrate 12 and the lower cladding layer 14.
Of such stacked structure, the upper cladding layer 18 and the contact layer 20 are formed as a mesa-structured portion having a ridge stripe pattern.
The both sides of the upper cladding layer 18 and the contact layer 20 composing the mesa-structured portion, and the upper cladding layer 18 are buried with an n-GaAs layer 22 provided as a current blocking layer to ensure current constriction, thereby a central portion of the active layer becomes an oscillation area 15 of laser light.
A metal layer made of Au, Ni and the like, or a metal stacked film is provided as a p-side electrode 24 on the n-GaAs layer 22 and the contact layer 20, and as an n-side electrode 26 on the rear surface of the GaAs substrate 12, respectively.
In order to fabricate such semiconductor laser device 10, at first the lower cladding layer 14, active layer 16, upper cladding layer 18 and contact layer 20 are epitaxially grown in this order on the GaAs substrate 12 by the metal-organic chemical vapor deposition (MOCVD) process.
The contact layer 20 and the upper cladding layer 18 are then etched to form the mesa-structured portion, and the n-GaAs layer 22 is then selectively grown on the both sides of the mesa-structured portion and on the upper cladding layer 18.
Next, the p-side electrode 24 and n-side electrode 26 are formed by, for example, the sputtering process on the outermost surface and on the rear surface of the GaAs substrate 12.
In the process of epitaxially growing the AlGaInP layer and the like to form the stack-structured portion, there has, however, been a problem of generating a growth defect in the epitaxially grown layer(s) if fine particles of GaAs or so adhere thereon, or foreign intermediate products are formed on the substrate during the epitaxial growth.
In the process of etching the stack-structured portion to form the mesa-structured portion after the epitaxial growth, etching with an acid of such epitaxially grown layer having the growth defect will result in formation of a pit-like shape defect portion 28 of several to tens tm diameter reaching the GaAs substrate 12 as shown in FIG. 7, since the portion of the growth defect is labile to acid and shows a high etch rate.
If the electrode layer 24 is formed in this situation, the electrode layer 24 intruded into the shape defect portion 28 will come into contact with the GaAs substrate 12 to cause short circuit. Such shape defect portion 28 can be produced in the stack-structured portion made of compound semiconductor layers not only during the wet etching but also during acid cleaning or alkali cleaning based on the same mechanism as described above.
As a result, short circuit will occur between currents injected to the both electrodes, thereby current which essentially has to be injected to the oscillation area in the active layer responsible for laser oscillation is reduced, and it causes non-emissive failures such that no laser oscillation occurs or the laser oscillation does not continue.
It is, however, quite difficult in practice in fabricating the semiconductor laser device to epitaxially grow the compound semiconductor layer after thoroughly cleaning the GaAs substrate and confirming that no particles adhering thereon. Thus so long as the semiconductor laser device is fabricated according to the conventional process, those suffering from non-emissive failures will be more or less produced to degrade the production yield.
It is therefore an object of the present invention to provide a semiconductor laser device having a structure capable of preventing non-emissive failure and a method for fabricating such device.
To accomplish such object, a semiconductor laser device comprises: a compound semiconductor substrate; a lower cladding layer; an active layer; an upper cladding layer and a contact layer respectively formed on the compound semiconductor substrate, wherein an upper part of the upper cladding layer and the contact layer are formed as a mesa-structured portion having a ridge stripe pattern,; and a current blocking layer having a pit-like recess penetrating thereof and extending towards the compound semiconductor substrate, the both sides of the mesa structured portion are buried with the current blocking layer, and a portion of the recess other than that penetrating the current blocking layer being covered or buried with an insulating film or a compound semiconductor layer with a high resistivity.
In the present invention, of the pit-like recess, a portion of which other than that penetrating the current blocking layer is covered or buried with an insulating film or a compound semiconductor layer with a high resistivity, so that the compound semiconductor substrate and the electrode layers other than the a current injection area are kept insulated, thereby the non-emissive failures as observed for the conventional semiconductor laser device is avoided.
The pit-like recess may not necessarily reach the compound semiconductor substrate and may be such that penetrating the current blocking layer to reach the upper cladding layer, active layer or lower cladding layer. It is also allowable that not only a portion of the recess other than that penetrating the current blocking layer, but also the entire part of the recess is covered or buried with an insulating film or a compound semiconductor layer with a high resistivity.
The current blocking layer is made of a compound semiconductor layer with a high resistivity, or a current blocking layer using a p-n junction isolation.
The present invention is applicable irrespective of compositions of the compound semiconductor substrate or compound semiconductor layers and, for example, preferably applicable to a semiconductor laser device with a laser oscillating structure composed of an AlGaInP-base or GaInP-base compound semiconductor layer formed on a GaAs substrate. The present invention is applicable to both semiconductor laser devices of edge-emitting type and surface-emitting type.
A structure responsible for the laser emission is not necessarily of the stacked structure comprising the lower cladding layer, active layer, upper cladding layer and contact layer, but also may be such structure that having a buffer layer between the substrate and the under cladding layer, or also may be such structure that having another layer such as a light confining layer between the contact layer and upper cladding layer.
In a preferred embodiment of the present invention, the insulating film may be made of at least any one of SiO2 film, Al2 O3 film and SiN film, a thickness of which being within a range from 100 nm to 50 xcexcm. The insulating film may be a stacked film thereof.
The insulating film may be made of a semi-insulating material doped or ion-implanted with boron. The compound semiconductor layer with a high resistivity may be made of a GaAs layer with a low carrier density of, for example, from 1xc3x971016/cm3 to 1 xc3x971018/cm3, both inclusive.
One method for fabricating such semiconductor laser device (referred as a first inventive method, hereinafter) relates to a method for fabricating a semiconductor laser device having on a compound semiconductor substrate at least a lower cladding layer, an active layer, an upper cladding layer and a contact layer; an upper part of the upper cladding layer and the contact layer being formed as a mesa structured portion having a ridge stripe pattern, and the both side of the mesa structured portion being buried with a current blocking layer, the method comprises steps of:
forming a stacked structure on a compound semiconductor substrate by epitaxially growing thereon a lower cladding layer, an active layer, an upper cladding layer and a contact layer in this order,
forming an insulating film on the entire surface of the substrate including the wall plane of a pit-like recess penetrating the current blocking layer and extending towards the compound semi conductor substrate,
forming a photoresist film on the entire surface of the substrate; patterning the photoresist film to form a resist mask on the insulating film as well as to fill the pit-like recess with the photoresist film,
etching the insulating film using the resist mask as an etching mask to form an insulating film mask, and then etching the contact layer and the upper cladding layer using the insulating film mask as an etching mask to form a mesa-structured portion having a ridge stripe pattern,
selectively growing, using the insulating film mask as a mask, a current blocking layer thereby to bury the both sides of the mesa-structured portion, and
removing the insulating film mask to expose the contact layer, and then forming an electrode layer on the surface of the substrate including on the contact layer.
Another method for fabricating such semiconductor laser device (referred as a second inventive method, hereinafter) relates to a method for fabricating a semiconductor laser device of an edge-emitting type having on a compound semiconductor substrate a lower cladding layer, an active layer, an upper cladding layer and a contact layer; an upper part of the upper cladding layer and the contact layer being formed as a mesa structured portion having a ridge stripe pattern, and the both side of the mesa structured portion being buried with a current blocking layer, the method comprises steps of:
forming a stacked structure on a compound semiconductor substrate by epitaxially growing thereon a lower cladding layer, an active layer, an upper cladding layer and a contact layer in this order,
etching the contact layer and the upper cladding layer to form a mesa-structured portion having a ridge stripe pattern,
selectively growing, using an insulating film mask, a current blocking layer thereby to bury the both sides of the mesa-structured portion,
removing the insulating film mask to expose the contact layer, and then forming an electrode layer on the surface of the substrate,
forming an insulating film on the entire surface of the substrate including the wall plane of a pit-like recess penetrating the current blocking layer and extending towards the compound semiconductor substrate, and then removing the insulating film from an area other than the wall plane of the pit-like recess, and
forming an electrode layer on the surface of the substrate including on the contact layer.
Still another method for fabricating such semiconductor laser device (referred as a third inventive method, hereinafter) relates to a method for fabricating a semiconductor laser device of an edge-emitting type having on a compound semiconductor substrate a lower cladding layer, an active layer, an upper cladding layer and a contact layer; an upper part of the upper cladding layer and the contact layer being formed as a mesa structured portion having a ridge stripe pattern, and the both side of the mesa structured portion being buried with a current blocking layer, the method comprises steps of:
forming a stacked structure on a compound semiconductor substrate by epitaxially growing thereon a lower cladding layer, an active layer, an upper cladding layer and a contact layer in this order,
etching the contact layer and the upper cladding layer to form a mesa-structured portion having a ridge stripe pattern,
selectively growing, using an insulating film mask, a current blocking layer with a low carrier density thereby to bury the both sides of the mesa-structured portion and a pit-like recess extending towards the compound semiconductor substrate, and then removing the insulating film mask to expose the contact layer, and
forming an electrode layer on the surface of the substrate including the contact layer.
Still further another method for fabricating such semiconductor laser device (referred as a fourth inventive method, hereinafter) relates to a method for fabricating a semiconductor laser device of an edge-emitting type having on a compound semiconductor substrate a lower cladding layer, an active layer, an upper cladding layer and a contact layer; an upper part of the upper cladding layer and the contact layer being formed as a mesa structured portion having a ridge stripe pattern, and the both side of the mesa structured portion being buried with a current blocking layer, the method comprises steps of:
forming a stacked structure on a compound semiconductor substrate by epitaxially growing thereon a lower cladding layer, an active layer, an upper cladding layer and a contact layer in this order,
etching the contact layer and the upper cladding layer to form a mesa-structured portion having a ridge stripe pattern,
selectively growing, using an insulating film mask, a current blocking layer thereby to bury the both sides of the mesa-structured portion, and then removing the insulating film mask to expose the contact layer,
forming a resist pattern on the contact layer, and performing ion implantation to the entire surface of the substrate thereby to convert the outermost surface of the wall plane of a pit-like recess penetrating the current blocking layer and extending towards the compound semiconductor substrate into a layer with a higher resistivity, and
removing the resist pattern thereby to form an electrode layer on the surface of the substrate including on the contact layer without annealing.
While there is no specific limitation on a method for forming the insulating film in the first to fourth inventive methods, the film is preferably formed by the chemical vapor deposition (CVD) process. The current blocking layer is formed by the metal-organic chemical vapor deposition (MOCVD) process.
There is no specific limitation on ion species in the fourth inventive method, and boron can be ion-implanted for example.
In the first, second and fourth inventive methods, the wall plane of the pit-like recess conceptually include a bottom plane of the recess, as well as a side plane thereof.
According to the present invention, in the process of fabricating the semiconductor laser device, at least a portion excluding such that penetrating the current blocking layer of the pit-like recess, occurred so as to penetrate the current blocking layer and to reach the compound semiconductor substrate, is covered or filled with the insulating film or the compound semiconductor layer with a higher resistivity, so that the compound semiconductor substrate and the electrode layer can be kept insulated in an area other than a current injection area, thereby non-emissive failure as has been observed in the conventional semiconductor laser device is preventted.
The method according to the present invention embodies a preferable method for fabricating the semiconductor laser device of the present invention.